Emergence of Classical Orbits in Few-Cycle Above-Threshold Ionization

The time-dependent Schr\"odinger equation for atomic hydrogen in few-cycle laser pulses is solved numerically. Introducing a positive definite quantum distribution function in energy-position space, a straightforward comparison of the numerical ab initio results with classical orbit theory is facilitated. Integration over position space yields directly the photoelectron spectra so that the various pathways contributing to a certain energy in the photoelectron spectra can be established in an unprecedented direct and transparent way.Comment: 4 pages, 4 figures REVTeX (manuscript with higher resolution figures available at http://www.dieterbauer.de/publist.html

Boosting Monte Carlo simulations of spin glasses using autoregressive neural networks

The autoregressive neural networks are emerging as a powerful computational tool to solve relevant problems in classical and quantum mechanics. One of their appealing functionalities is that, after they have learned a probability distribution from a dataset, they allow exact and efficient sampling of typical system configurations. Here we employ a neural autoregressive distribution estimator (NADE) to boost Markov chain Monte Carlo (MCMC) simulations of a paradigmatic classical model of spin-glass theory, namely the two-dimensional Edwards-Anderson Hamiltonian. We show that a NADE can be trained to accurately mimic the Boltzmann distribution using unsupervised learning from system configurations generated using standard MCMC algorithms. The trained NADE is then employed as smart proposal distribution for the Metropolis-Hastings algorithm. This allows us to perform efficient MCMC simulations, which provide unbiased results even if the expectation value corresponding to the probability distribution learned by the NADE is not exact. Notably, we implement a sequential tempering procedure, whereby a NADE trained at a higher temperature is iteratively employed as proposal distribution in a MCMC simulation run at a slightly lower temperature. This allows one to efficiently simulate the spin-glass model even in the low-temperature regime, avoiding the divergent correlation times that plague MCMC simulations driven by local-update algorithms. Furthermore, we show that the NADE-driven simulations quickly sample ground-state configurations, paving the way to their future utilization to tackle binary optimization problems.Comment: 13 pages, 14 figure

Enhancement of bichromatic high-harmonic generation with a high-frequency field

Using a high-frequency field superposed to a linearly polarized bichromatic laser field composed by a wave with frequency $\omega$ and a wave with frequency $2\omega$, we show it is possible to enhance the intensity of a group of high harmonics in orders of magnitude. These harmonics have frequencies about 30% higher than the monochromatic-cutoff frequency, and, within the three-step-model framework, correspond to a set of electron trajectories for which tunneling ionization is strongly suppressed. Particular features in the observed enhancement suggest that the high-frequency field provides an additional mechanism for the electron to reach the continuum. This interpretation is supported by a time-frequency analysis of the harmonic yield. The additional high frequency field permits the control of this group of harmonics leaving all other sets of harmonics practically unchanged, which is an advantage over schemes involving only bichromatic fields.Comment: 6 pages RevTex, 5 figures (ps files), Changes in text, figures, references and equations include

Ellipticity of High-Order Harmonics Generated by Aligned Homonuclear Diatomic Molecules Exposed to an Orthogonal Two-Color Laser Field

We investigate emission rate and ellipticity of high-order harmonics generated exposing a homonuclear diatomic molecule, aligned in the laser-field polarization plane, to a strong orthogonally polarized two-color (OTC) laser field. The linearly polarized OTC-field components have frequencies r? and s?, where r and s are integers. Using the molecular strong-field approximation with dressed initial state and undressed final state, we calculate the harmonic emission rate and harmonic ellipticity for frequency ratios 1:2 and 1:3. The obtained quantities depend strongly on the relative phase between the laser-field components. We show that with the OTC field it is possible to generate elliptically polarized high-energy harmonics with high emission rate. To estimate the relative phase for which the emission rate is maximal we use the simple man’s model. In the harmonic spectra as a function of the molecular orientation there are two types of minima, one connected with the symmetry of the molecular orbital and the other one due to destructive interference between different contributions to the recombination matrix element, where we take into account that the electron can be ionized and recombine at the same or different atomic centers. We derive a condition for the interference minima. These minima are blurred in the OTC field except in the cases where the highest occupied molecular orbital is modeled using only s or only p orbitals in the linear combination of the atomic orbitals. This allows us to use the interference minima to assess which atomic orbitals are dominant in a particular molecular orbital. Finally, we show that the harmonic ellipticity, presented in false colors in the molecular-orientation angle vs. harmonic-order plane, can be large in particular regions of this plane. These regions are bounded by the curves determined by the condition that the harmonic ellipticity is approximately zero, which is determined by the minima of the T-matrix contributions parallel and perpendicular to the fundamental component of the OTC field. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Nondipole effects in terahetz-pulse-assisted strong-field ionization

Nondipole effects in processes assisted by a THz field having the strength of a few MV/cm can be significant due to its long wavelength. We illustrate this for strong-laser-field-induced ionization assisted by a THz field. To this end, we generalize our strong-field-approximation theory so that it includes the first-order term in a 1/c expansion of the vector potential. We show that in this case, in addition to a shift of the maximum of the photoelectron momentum distribution, the differential ionization probability as well as the cutoff energy can be significantly increased. For an explanation of these unexpected results we use the saddle-point method adjusted to include nondipole effects

Fluctuations in superconducting rings with two order parameters

Starting from the Ginzburg-Landau energy functional, we discuss how the presence of two order parameters and the coupling between them influence a superconducting ring in the fluctuative regime. Our method is exact, but requires numerical implementation. We also study approximations for which some analytic expressions can be obtained, and check their ranges of validity. We provide estimates for the temperature ranges where fluctuations are important, calculate the persistent current in magnesium diboride rings as a function of temperature and enclosed flux, and point out its additional dependence on the cross-section area of the ring. We find temperature regions in which fluctuations enhance the persistent currents and regions where they inhibit the persistent current. The presence of two order parameters that can fluctuate independently always leads to larger averages of the order parameters at Tc, but only for appropriate parameters this yields larger persistent current. In cases of very different material parameters for the two coupled condensates, the persistent current is inhibited

Mesoscopic field and current compensator based on a hybrid superconductor-ferromagnet structure

A rather general enhancement of superconductivity is demonstrated in a hybrid structure consisting of submicron superconducting (SC) sample combined with an in-plane ferromagnet (FM). The superconducting state resists much higher applied magnetic fields for both perpendicular polarities, as applied field is screened by the FM. In addition, FM induces (in the perpendicular direction to its moment) two opposite current-flows in the SC plane, under and aside the magnet, respectively. Due to the compensation effects, superconductivity persists up to higher applied currents. With increasing current, the sample undergoes SC-"resistive"-normal state transitions through a mixture of vortex-antivortex and phase-slip phenomena.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let